Ah, Now I understand what you're getting at!
Your reasons for concern are valid and I thank you for pointing out a possible issue.
Having re-evaluated the design and discussed with my engineer/welder friend i’ve come to the conclusion that if there were only 2 spreading bars I would agree that more triangulation would need to be added. The design has four connecting struts to front and rear of the fork all of which are angled to transmit the loads.
Look at this Vincent girder fork for example; which has only 2 legs. Instead of round tubes it utilises a thick plate to perform a similar function. Also note the material (mild steel) and section/thickness of the fork legs compared to my Hossack design.
I will be keeping a close eye on all the frame and forks for any signs of fatigue once testing begins. My sense of life preservation is VERY strong!